Benefits of mechanized deep placement of nitrogen fertilizer in direct-seeded rice in South China

Compared to surface broadcasting, mechanically deep placement of N fertilizer enhanced NUE and grain yield significantly while improved nitrogen recovery efficiency (NRE) by 32.52-50.79%, 21.51-32.68%, and nitrogen agronomy efficiency (NAE) by 32.10-50.43%, 19.47-38.78%, of Tianyou 998 and Yuxiangyouzhan, respectively, in both seasons. Compared to surface broadcasting, the grain yields of Tianyou 998 and Yuxiangyouzhan under mechanically deep placement of N fertilizer were also increased by 6.31-8.03% and 4.70-6.78%, respectively, which were ascribed to the increased spikelet number per panicle. Furthermore, the lower production cost and the higher total return in CFM led to the highest benefit-cost ratio (BCR) in CFM, followed by CFB and ABM, whereas the lowest BCR was recorded for ABB. In addition, significant improvements were also observed in leaf area index, total aboveground biomass, and photosynthesis rate in CFM at the heading stage. Catalase (CAT) and Peroxidase (POD) activities of the uppermost leaves were significantly higher under deep placement of N fertilizer than surface broadcasting. Hence, mechanized deep placement of commercial compound fertilizer can be taken as an efficient fertilization method in mechanical hill direct-seeded rice in South China.